A urinary tract infection is an infection involving the organs that produce urine and carry it out of the body. These structures include the kidneys, ureters (long, slender tubes connecting the kidneys with the bladder), bladder and urethra. Doctors often divide urinary tract infections into two types, lower tract infections and upper tract infections:
Lower tract infections — Infection of the bladder is called cystitis (bladder infection). Bacteria normally found in the intestine are the main cause of lower urinary tract infections. These bacteria spread from the anus to the urethra and bladder, where they grow, invade the tissue and cause infection.
Upper tract infections — These involve the ureters and kidneys. These infections are called pyelonephritis or kidney infections. Upper urinary tract infections usually occur because bacteria travel up from the bladder into the kidney. Sometimes, they occur when bacteria travel from other areas of the body through the bloodstream and settle in the kidney.
Women are affected much more often than men because women have short urethras that allow relatively easy passage of bacteria into the bladder. Sexual intercourse can cause bacteria to spread upward into the bladder. Also, the use of contraceptive diaphragms and spermicides may change the normal bacterial environment around the urethra and make infection more likely.
In pregnant women, temporary changes in the physiology and anatomy of the urinary tract make expectant mothers prime candidates for cystitis and pyelonephritis. Kidney and bladder infections can pose a serious risk to pregnant women and their unborn children, because they increase the risk of premature contractions or delivery and sometimes death of the fetus or newborn infant.